1. Field of the Invention
The present invention relates to a single lens usable as an objective lens which is used in an optical pickup of a recording and reproducing apparatus for an optical recording medium such as a video disk, an audio disk, an optical memory disk, and the like, and more particularly to a single lens usable as an objective lens which is capable of converging light from a light source without using a collimator lens.
2. Description of the Prior Art
The objective lens used in an optical pickup of an optical disk recording and reproducing apparatus is required to precisely correct abberations to its diffraction limit, to provide a wide angle of view, to be cheap, and to allow the optical pickup to be as compact as possible. The conventional objective lens, which is made by combining a plurality of spherical glass lenses to fully satisfy the desired optical performance, cannot satisfy those requirements mentioned above.
On the other hand, optical pickups using a single aspherical lens have been proposed by Japanese Patent Laid-Open No. 57-76512 (1982), Japanese Patent Laid-Open No. 57-201210 (1982), Japanese Patent Laid-Open No. 59-26714 (1984), Japanese Patent Laid-Open No. 59-23313 (1984), and Japanese Patent Laid-Open No. 60-120310 (1985), respectively. However, each of these pickups disclosed by these prior arts converts light from a light source into parallel beams by a collimator lens before converging the parallel beams onto the back surface of a disk by the objective lens.
These pickups would be expensive due to the use of a number of lenses, and cannot be made compact due to the necessity of a space for the collimator lens.
Japanese Patent Laid-Open No. 50-156945 (1975) discloses a finite-conjugate aspherical single lens. However, the magnification of this lens is only 1/20, which is very small, and yet, since there is a long distance between the light source and the light-condensing point, compact size cannot easily be realized for the optical pickup. Furthermore, when combining this lens with a laser diode, which is widely used for the light source, the numerical aperture of the lens toward the light-source end is too small to efficiently use the effective quantity of light from the light source.
Japanese Patent Laid-Open No. 60-126616 (1985) proposes a single lens capable of converging a light from a light source. However, taking the manufacturing convenience into account, only one of the two surfaces of the single lens is made aspherical, and as a result, the lens cannot provide a wide field angle. Consequently, in order to effectively use this single lens in the optical pickup, the light source and the lens must be housed in a tubular housing which is movable for tracking. This structure does not allow the optical pickup to be made compact, and in addition, due to its heavy weight, power-saving cannot be realized.
Japanese Patent Laid-Open No. 61-56314 (1986) proposes a bi-aspheric single lens which is capable of satisfying optical performances to the extent of its diffraction limit even when moved independently without being integrally moved with the light source. However, in order to achieve a compact optical pickup, it is essentially required to insert a mirror and a prism in the optical path from the light source to the lens. As a result, in consideration of mounting errors of the mirror, prism, and light source, and also in consideration of the movement of the lens in the tracking direction, a still wider field angle is needed.